Spectrometer designs

ABSTRACT

Various embodiments include spectrometers comprising diffraction gratings monolithically integrated with other optical elements. These optical elements may include slits and mirrors. The mirrors and gratings may be curved. In one embodiment, the mirrors are concave and the grating is convex. The mirrors and grating may be concentric or nearly concentric.

PRIORITY APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 11/141,355 filed on May 31, 2005 entitled“Spectrometer Designs”, which is considered part of, and is incorporatedherein by reference in its entirety. This application also claimspriority to U.S. Provisional Patent Application No. 60/685,217 filed May27, 2005, entitled “Spectrometer Designs” (Attorney Docket No.NOSOL.002PR), which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The invention described herein relates to spectrometers including, forexample, spectrometers having gratings monolithically integrated withother optical elements.

2. Description of the Related Art

Spectrometers are optical instruments that determine the spectralcontent of an optical signal. The output of a spectrometer is a spectraldistribution of intensity versus wavelength, referred to herein as aspectrum.

Spectrometers are very useful in a myriad of scientific andtechnological applications and are the basis of spectroscopy.Spectroscopy may, for example, help identify the composition ofmaterials and may provide information regarding different physical andchemical processes.

Imaging spectrometers are a special type of spectrometer that produceswavelength spectrums for different spatial locations in atwo-dimensional field. Imaging spectroscopy can be accomplished byproducing spectrums for a plurality of sites along one swath of thetwo-dimensional field. A recording device, such as an array ofphotodetectors, is located at an image plane to record the spectralinformation for locations across the swath. The instrument is then sweptover the next swath and the spectral response of each portion of the newswath is measured in a like manner. Spectroscopic information canthereby be obtained for a two-dimensional array of locations.

Spectrometers are used both in the laboratory and in the field. Forvarious applications imaging spectrometers may be included as payloadsin satellites, airplanes, or unmanned aerial vehicles (UAVs). Suchspectrometers may be used, for example, for remote sensing andreconnaissance. In the case of imaging spectrometers on satellite andairplane platforms, the instrument can be scanned over thetwo-dimensional field by the motion of the platform itself. In this way,a map over the spectral response of the entire two-dimensional field canbe created.

Two characteristics of spectrometers that are therefore desirable arerigidity and small size. Rigidity can be important to ensure that theinstrument maintain precise alignment of optical components to achievedesired performance. Over its lifetime the instrument can be subjectedto vibration and other physical stresses that can degrade instrumentperformance if proper alignment of the optical components is lost. Thesetypes of physical stresses can occur during rocket launch of a satellitepayload or during turbulence or during maneuvering and landing of anairplane or UAV, for example. Small size is also important because spaceis generally limited for airplane and UAV based instruments andespecially in the case of satellite missions where extra size and weightcan add significantly to the cost of placing the satellite in orbit.There is a need, therefore, for a spectrometer with increased ruggednessand decreased size.

SUMMARY

One embodiment of the invention comprises a spectrometer comprising: afirst body portion comprising substantially optically transmissivematerial; first and second reflective regions disposed on a first sideof said first body portion; a reflective grating disposed on a secondside of said first body portion; and a second body portion comprisingsubstantially optically transmissive material joined to said first bodyportion with said reflective grating disposed therebetween, wherein saidfirst and second reflective regions and said reflective grating arearranged with respect to each other such that light incident on saidfirst reflective region is reflected to said grating, diffracted fromsaid grating to said second reflective portion, and reflected from saidsecond reflective portion into said second body portion.

Another embodiment of the invention comprises a spectrometer comprising:a body comprising a mass of substantially optically transmissivematerial; a first reflector; a curved reflective grating, said firstreflector and said curved reflective grating defining a first opticalpath therebetween, said first reflector and said reflective gratingdisposed with respect to said body such that said first optical pathsubstantially comprises said substantially transmissive material; and adetector defining a second optical path extending from said curvedreflective grating to said detector, said second optical pathsubstantially comprising said substantially transmissive material.

Another embodiment of the invention comprises a spectrometer comprising:a body comprising a mass of substantially optically transmissivematerial; a first curved reflector; and a reflective grating, said firstcurved reflector and said reflective grating defining a first opticalpath therebetween, said first curved reflector and said reflectivegrating disposed with respect to said body such that said first opticalpath substantially comprises said substantially transmissive material,wherein said reflective grating is configured to reflect broadband lighthaving a bandwidth of at least about 400 nanometers to a detector via asecond optical path comprising said substantially transmissive material.

Another embodiment of the invention comprises a spectrometer comprising:a first body portion comprising substantially optically transmissivematerial; first and second reflective regions disposed on a first sideof said first body portion; and a reflective grating disposed on asecond side of said first body portion; wherein said first and secondreflective regions and said reflective grating are arranged with respectto each other such that broadband light at least about 300 nanometers inbandwidth propagating through said substantially optically transmissivematerial incident on said first reflective region is reflected to saidgrating, diffracted from said grating through said substantiallyoptically transmissive material to said second reflective region, andreflected from said second reflective region through said opticallytransmissive material.

Another embodiment of the invention comprises a spectrometer comprising:a medium comprising substantially optically transmissive material; aslit; and a reflective grating, said slit and said reflective gratingdefining a first optical path therebetween, said slit and saidreflective grating disposed with respect to each other and said mediumsuch that said first optical path substantially comprises saidsubstantially transparent material.

Another embodiment of the invention comprises a spectrometer configuredto be mounted in an unmanned airborne vehicle, the spectrometercomprising: a body comprising a mass of substantially opticallytransmissive material; a first reflector; a reflective grating, saidfirst reflector and said reflective grating defining a first opticalpath therebetween, said first reflector and said reflective gratingdisposed with respect to said body such that said first optical pathsubstantially comprises said substantially transmissive material, saidreflective grating being configured to reflect light to a detector via asecond optical path comprising said substantially transmissive material;and a housing in which said body, said first reflector, and saidreflective grating are positioned, wherein said housing is no greaterthan 1000 cubic centimeters, and said housing is configured to bemounted in an unmanned airborne vehicle receiving area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one embodiment of a spectrometercomprising a convex diffraction grating and a pair of concave mirrorsintegrated together in a monolithic structure;

FIG. 2 is a schematic view of an example spectrometer entrance slit;

FIG. 3 is a schematic view of light incident upon the entrance slit ofan example spectrometer where the propagation medium on either side ofthe slit has the same index of refraction;

FIG. 4 is a schematic view of light incident upon the entrance slit ofan example spectrometer comprising material having a high index ofrefraction that causes the light entering the spectrometer through theslit to have a reduced cone angle within the instrument; and

FIG. 5 is a schematic view illustrating one configuration where opticalcomponents comprising concentric surfaces are used to producewell-corrected images relatively free of optical aberrations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates one embodiment of an imagingspectrometer 100. The spectrometer instrument 100 comprises a generallymonolithic assemblage of optical components that are integrated togetherusing a rigid support structure or main body 110 comprisingsubstantially optically transmissive material. As shown in FIG. 1 anddiscussed more fully below, light collected by the example spectrometer100 propagates through the optically transmissive material comprisingthe support structure 110 interacting with optical components integratedthereon and/or therein.

In this example, these optical components include an entrance slit 112,a reflective surface 114, first and second curved mirrors 116, 118, adiffraction grating 120, and a sensor 122. The entrance slit 112 isdisposed on a turn mirror block 124 that includes the reflective surface114. The first and second curved mirrors 116, 118 and diffractiongrating 120 are disposed on different sides of a meniscus block 126.This meniscus block 126 has first and second curved surfaces 128, 130.The first and second curved mirrors 116, 118 are disposed on the firstcurved surface 128 and the grating 120 is disposed on the second curvedsurface 130.

The spectrometer instrument 100 further comprises a plano-convex block132 having a first curved surface 134 and a second flat surface 136. Thefirst curved surface 134 on the plano-convex block 132 mates with thesecond curved surface 130 on the meniscus block 126. The turn block 124also has a planar surface 138 that is butt up against the second flatsurface 136 of the plano-convex block 132. The spectrometer 100 furthercomprises an output block 140 having first and second planar surfaces142, 143. The first planar surface 142 on the output block 140 is buttup against the second flat surface 136 of the plano-convex block 132. Asubstantially optically transmissive adhesive may be used to attach theturn block 124 and the output block 140 to the plano-convex block 132and the plano-convex block to the meniscus block 126. In variouspreferred embodiments, the substantially optically transmissive adhesivehas an index of refraction substantially matching that of the turn block124, the output block 140, the plano-convex block 132, and the meniscusblock 126 to reduce Fresnel reflection.

In certain preferred embodiments, the spectrometer is substantiallycompact. For example, the monolithic structure 110 comprising the turnblock 124, the output block 140, the plano-convex block 132, and themeniscus block 126 bonded together with adhesive may have a length, L,as shown in FIG. 1, between about 25 millimeters (mm) and 75 mm and awidth, W, as shown, between about 20 mm and 60 mm. The thickness, notshown in FIG. 1, may be between about 10 mm and 30 mm. Discussionsoutside these ranges are also possible. In one embodiment, themonolithic structure 110 has dimensions of approximately 50 mm long, 40mm wide, and 22 mm high, although the structure may be larger orsmaller. In certain embodiments, the spectrometer 100 is also light. Themonolithic structure comprising the turn block 124, the output block140, the plano-convex block 132, and the meniscus block 126 togethermay, for example, weigh between about 50 grams and 1000 grams, but maybe heavier or lighter in other embodiments.

The spectrometer 100 further comprises imaging optics 144 shown in blockdiagram form in FIG. 1. An optical path extends from the imaging optics144 to the slit 112. The spectrometer 100 also may comprise a processoror computer 146 configured to collect images from the sensor 122. Thisprocessor 146 may comprise electronics electrically connected to thesensor 122, which may comprise a two-dimensional detector array such asa CMOS or CCD solid state detector array or a HgCdTe or InSb solid statedetector array. The image sensor 122 may be included in a package andmay optionally be cooled by a cooler.

In certain preferred embodiments, light propagates along an optical paththrough the instrument 100 substantially as follows. Light emanates orreflects from a remote object (not shown) and is collected by theimaging optics 144. The remote object can be, for example, the groundbelow in a UAV application. The imaging optics 144, which representsoptics suitable for conditioning light to serve as the input to theinstrument 100, forms an image of the remote object onto the entranceslit 112 of the spectrometer instrument 100. The light passes throughthe entrance slit 112, diverging, and enters the optically transmissivemedium which comprises the majority of the optical path through theinstrument 100. The emerging light beam is directed toward thereflective surface 114 in the turn block 124. This reflective surface114 is oriented to redirect the light about 90° in the embodiment shownin FIG. 1 through the plano-convex block 132 and the meniscus block 126to the first mirror 116. The diverging light beam reflects from thefirst mirror 116, a concave mirror that produces a converging beam.

The first mirror 116 directs the light through the substantiallytransmissive material in the meniscus block 126 to the grating 120. Thegrating 120 is a convex surface that converts the converging beam fromthe first mirror 116 into a diverging beam. The grating 120 alsodiffracts the beam. One order of the diffracted beam is directed to thesecond mirror 118. The grating 120 also introduces dispersion such thatdifferent wavelengths are diffracted at distinct angles therebyspatially separating the different wavelength components. The diffractedbeam propagates through the substantially optically transmissivematerial in the meniscus block 126 to the second mirror 118, whichcomprises a concave reflecting surface. Accordingly, the divergentdiffracted beam is converted into a convergent beam directed through thesubstantially optically transmissive material in the meniscus block 126,the plano-convex block 132, and the output block 140. After exiting theoutput block 140, the light strikes the sensor 122, which convertsoptical energy across the spatial extent of the sensor into electricalsignals which may be recorded and/or processed. The sensor 122, atwo-dimensional detector array, produces signals indicative of thespatial distribution of the intensity on the sensor. These signals areconveyed to the computer 146 for image processing.

In certain preferred embodiments, the spectrometer is configured forlight having a broad bandwidth, e.g., about 200, 300, or 400 nanometers(nm) or more. For example, certain embodiments are configured forvisible light having a wavelength between about 400-800 nm or 450-900nm. The design (e.g., shape, size, materials, etc.) and location of theoptical components (e.g., grating, reflectors, detector, etc.) may beselected such that broadband light can be processes by the spectrometer100. Larger or smaller bands are also possible.

The imaging optics 144 may comprise one or more lens elements in certainembodiments. For example, the imaging optics 144 may comprise a lens orlens system similar to that used in a camera. Zoom or wide field opticsmay be employed. The imaging optics 144 may have a focal length betweenabout 20-100 millimeters (mm) and an f-number between about 2 and 4 insome embodiments, although values outside these ranges are possible. Theimaging optics 144 are disposed with respect to the slit 112 to form animage on the slit. This image may, for example, be about 5 mm to 25 mmwide and high in certain embodiments, and the imaging optics 144 may bea distance, e.g., between about 5 mm and 100 mm from the slit. The typeand configuration of the imaging optics, however, is not limited asother types of imaging optics and other designs can be employed. Theimage formed on the slit 112 will also vary in size and shape.

An exemplary slit 112 is depicted in FIG. 2. The slit 112 may have aheight, a, between about 5 mm and 25 mm, e.g., about 10 mm, and a width,b, between about 5 microns and 50 microns, e.g., about 10 microns. Moregenerally, the slit 112 is an aperture through which light enters thespectrometer 100. This aperture 112 may have different shapes anddifferent dimensions as well. In certain preferred embodiments where thespectrometer 100 comprises an imaging spectrometer, the slit 112 iselongated so as to pass light corresponding to an elongated portion ofthe image field, referred to above as a swath. In some embodiments,however, the aperture 112 may not be elongated and may, e.g., be a pointaperture comprising a small round hole. Such apertures may be used fornon-imaging spectrometers, for example.

The aperture 112 may comprise an opening in a mask 148 comprising, forexample, metal or other opaque material. This metal may be deposited ona front surface of the turn block 124; see FIG. 1. The mask 148 andaperture 112 may be formed using photolithographic techniques. In otherembodiments, the mask 148 may be attached to the front surface of theturn block 124, for example, by adhesive. Other types of apertures 112may be used and the aperture may be secured to the spectrometer 100 inother ways. The aperture 112 may also be located elsewhere. For example,in embodiments that do not employ the turn block 124, the aperture 112may be disposed on the surface 136 of the plano-convex block 132.Alternatively, the aperture 112 may be separate from the main body 110of the spectrometer 100. Other configurations are also possible.

The turn block 124, shown in FIG. 1, may comprise substantiallyoptically transmissive material such as, for example, glass. Fusedsilica may be employed in certain preferred embodiments. Other materialsmay be employed as well. The front face of the turn block 124 may bepolished. As described above, the aperture 112 can be disposed on thisfront face. The turn block 124 may further be polished to form thereflective surface 114. Light propagating from the slit 112 through theturn block 124 may be reflected from this reflective surface 114 bytotal internal reflection. Alternatively, the reflective surface 114 maycomprise a reflective material such as metal or a dielectric reflector.Other designs are possible.

As discussed above, the reflective surface 114 may be oriented to directthe light propagated through the slit 112 along a path toward the firstmirror 116. This reflective surface 114 may therefore be oriented atabout 90° in some embodiments; however, the orientation may varydepending on the configuration and design.

The turn block 124 may assist in the packaging and arrangement of thecomponents. For example, the turn block 124 may allow the imaging optics144 to be farther away from the sensor 122. In certain embodiments,however, the turn block 124 and/or the reflective surface 114 may beexcluded. For example, the light beam from the slit 112 may directlypropagate to the first mirror 116 without being redirected by areflective element.

As demonstrated by FIGS. 3 and 4, the substantially opticallytransmissive material of the turn block 124 and other componentscomprising the main body 110 through which the light propagates in thespectrometer may advantageously enable a more compact design. Thesecomponents include, for example, the plano-convex block 136 and themeniscus block 126 shown in FIG. 1.

In particular, the substantially optically transmissive material mayhave a higher index of refraction than that of the environmentsurrounding the spectrometer 100 such that light is refracted uponentering the spectrometer 100. A comparison is presented in FIGS. 3 and4 of the cases where light propagates through air within thespectrometer 100 and light propagates through a medium having a higherrefractive index in the spectrometer 100. In particular, FIG. 3 showslight propagating through a region 150 comprising air and passingthrough an aperture 152 into a region 154 also comprising air. Norefraction is present as the medium is the same on both sides of theaperture 152. In contrast, FIG. 4 shows light propagating through aregion 156 comprising air and passing through an aperture 158 into aregion 160 comprising material such as glass, which has a higherrefractive index than air. The light is refracted. FIG. 4 shows how acone of light rays 162 passing through the aperture 158 has a smallersize within the material than in the air. This reduction in the size ofthe cone of rays 162 enables optics having a smaller size to be used inthe spectrometer design.

As shown in FIG. 1, a cone of rays entering the spectrometer 100 throughthe slit 112 is refracted in the turn block 124 comprising asubstantially optically transmissive material having a higher refractiveindex. The resultant cone of rays in the turn block 124 and similarly inthe spectrometer 100 comprising the substantially optically transmissivematerial is likewise reduced. Smaller optics can therefore be used.Accordingly, the optics within the spectrometer 100 may have a higherf-number (or reduced numerical aperture). Nevertheless, the spectrometer100 collects a larger cone of light equivalent to a smaller f-number (orincreased numerical aperture). A compact spectrometer design thatcollects more light can thus be achieved by propagating the light withinthe spectrometer 100 through a medium having a higher refractive indexthan the medium outside the spectrometer, which will likely comprise airor vacuum. Accordingly, the light is propagated through the main body110 in the spectrometer 100, which comprises a substantially opticallytransmissive material such as glass, which has an index of refraction ofabout 1.5 in some cases.

As discussed above in connection with FIG. 1, the main body 110 of thespectrometer 100 further comprises the plano-convex block 132. Thisplano-convex block 132 comprises substantially optically transmissivematerial, such as glass, having an index of refraction greater than airor vacuum. The plano-convex block 132 may also comprise fused silica incertain preferred embodiments. The plano-convex block 132 may bepolished to provide the first curved surface 134 and a second flatsurface 136. The curved surface 134 may be spherically shaped. Thedistance separating the first curved surface 134 and a second flatsurface 136 at the center may be between about 5 mm and 20 mm. Thisplano-convex block 132 may have a width between about 20 mm and 60 mm.The radius of curvature of the first curved surface 134 may be betweenabout 15 mm and 30 mm and may match the curvature of the grating 120 incertain preferred embodiments. Other dimensions can also be used.

As discussed above, the planar surface 138 of the turn block 124 is buttup against the second flat surface 136 of the plano-convex block 132.Light from the slit 112 thus is reflected by the reflective surface 114on the turn block 124 into the plano-convex block 132. This lightpropagates through the higher index material comprising the plano-convexblock 132.

Adhesive may be used to bond the planar surface 138 on the turn block124 to the second flat surface 136 on the plano-convex block 132. Thisadhesive may have an index of refraction substantially similar to thesubstantially optically transmissive material comprising the turn block124 and the plano-convex block 132. Such index matching may reduceFresnel reflections.

As discussed above in connection with FIG. 1, the main body 110 of thespectrometer 100 further comprises the meniscus block 126. This meniscusblock 126 also comprises substantially optically transmissive material,such as glass, having an index of refraction greater than air or vacuum.The meniscus block 126 may comprise fused silica in certain preferredembodiments. The meniscus block 126 may be polished to provide the firstand second curved surfaces 128, 130. These curved surfaces 128, 130 maybe rotationally symmetrical, and more particularly, spherically shaped.The distance separating the first and second curved surfaces 128, 130 atthe center (e.g., through the optical axis) may be between about 10 mmand 30 mm. This concave-convex block 126 may have a width between about20 mm and 60 mm. The radius of curvature of the first curved surface 128may be between about 30 mm and 60 mm and may have the same center ofcurvature of the second curved surface 130 as discussed more fullybelow. The curvature of the second curved surface 130 may be betweenabout 15 mm and 30 mm and may be about one-half the radius of curvatureof the first curved surface 128 in some embodiments also discussedbelow. Other dimension can be used as well. In certain embodiments, theradius of curvature of the second curved surface 130 matches thecurvature of the grating 120 in certain embodiments.

The first and second curved mirrors 116, 118 are disposed on the firstcurved surface 128 and the grating 120 is disposed on the second curvedsurface 130. The first and second curved mirrors 116, 118 may be formed,for example, by metallizing portions of the first curved surface 128 onthe meniscus block 126. A metallized region may extend through andinclude both the first curved mirror 116 and the second curve mirror118. In other embodiments, separated regions of metallization may beused to form the first and second curved mirrors 116, 118. Dielectriccoatings may be employed in some embodiments.

Other methods of forming mirrors on the first curved surface 128 mayalso be used. In some embodiments, the first and/or second mirrors 116,118 are mounted proximal to, possibly spaced apart from, the firstcurved surface 128. Index matching material may be used in suchembodiments to reduce reflection. In some embodiments, adhesive havingsuitable refractive index to reduce Fresnel reflection may be employedto adhere the mirrors 116, 118 to the first curved surface 128. Stillother methods may be used to provide reflective surfaces near the firstcurved surface.

The curved grating 120 may be formed on the second curved surface 130 ofthe meniscus block 126, for example, using photolithographic techniques.For example, metal may be deposited on the second curved surface 130 andpatterned using, e.g., photoresist. Other approaches may be employed.The grating 120 comprises a holographic optical element formed byholographic techniques. In some embodiments, low diffractive orders(e.g., n=1, etc.) are used. Low diffractive orders such as n=2, 3, 4, or5 could also be used. The low order may, for example, be directed to thesecond mirror 118 and conveyed to the sensor 122. Other orders may alsobe used.

In some embodiments, the grating 120 is mounted proximal to, possiblyspaced apart from, the second curved surface 130. Index matchingmaterial may be used in such embodiments to reduce reflection.Accordingly, in some embodiments, the first and second curved mirrors116, 118 can be spaced apart from (but disposed on a first side of) themeniscus block 126 and the grating 120 can be spaced apart from (butdisposed on a second side of) the meniscus block. Still otherconfigurations are possible.

As discussed above, the plano-convex block 132 is mated with themeniscus block 126. The first curved surface 134 on the plano-convexblock 132 may be butt up against the second curved surface 130 on themeniscus block 126. Adhesive may be used to bond the two surfaces 134,130 together. This adhesive may have an index of refractionsubstantially similar to the substantially optically transmissivematerial comprising the meniscus block 132 and the plano-convex block132. Such index matching may reduce Fresnel reflections.

Accordingly, light from the slit 112 is reflected by the reflectivesurface 114 on the turn block 124 into the plano-convex block 132 andthrough the substantially optically transmissive material in themeniscus block 126. The light reaches the first mirror 116 where thelight is reflected back through the substantially optically transmissivematerial in the meniscus block 126 to the diffraction grating 120. Thislight is diffracted by the reflective grating 120 and is directed onceagain through the substantially optically transmissive materialcomprising the meniscus block 126 to the second mirror 118. The light isreflected from the second mirror 118 again through the opticallytransmissive material in the meniscus block 126 and proceeds into theplano-convex block 132. Accordingly, the light propagates substantiallythrough high index material between the slit 112 and the first mirror114, the first mirror and the grating 120, the grating and the secondmirror 118 and to the plano-convex block 132. The light also propagatesthrough high index material within the plano-convex block 132.

As discussed above in connection with FIG. 1, the main body 110 of thespectrometer 100 also comprises the output block 140. This output block140 similarly comprises substantially optically transmissive material,such as glass, having an index of refraction greater than air or vacuum.The output block 140 may comprise fused silica in certain preferredembodiments. The output block 140 may be polished to provide first andsecond planar surfaces 142, 143. The distance separating the first andsecond planar surfaces 142, 143 at the center may be between about 5 mmand 20 mm. This output block 140 may have a width between about 10 mmand 30 mm. In certain preferred embodiments, the length of the outputblock 140 is sufficiently large such that a substantial portion of thepath from the second mirror 118 to the sensor 122 comprises high indexmaterial. The width of the optical block 140 may also be at least aslarge to accommodate the width of the beam from the second mirror 118 tothe sensor 122 in certain embodiments. Other sizes and shapes for theoutput block 140 are also possible.

As discussed above, the output block 140 is mated with the plano-convexblock 132. The first planar surface 142 of the output block 140 may bebutt up against the second planar surface 136 of the plano-convex block132. Adhesive may be used to bond the two surfaces 142, 136 together.This adhesive may have an index of refraction substantially similar tothat of the substantially optically transmissive material comprising theplano-convex block 132 and the output block 140. Such index matching mayreduce Fresnel reflections.

The spectrometer 100 further comprises the sensor 122 as discussedabove. The sensor 122 may comprise a detector array comprising atwo-dimensional array of detectors or pixels. Such a sensor 122 maycomprise a CMOS detector or a CCD detector. Other types of detectors mayalso be used. For example, the sensor 112 may comprise mercury cadmiumtelluride (HgCdTe) or indium antimonide (InSb). Still other types ofsensors are possible. The sensor 122 may be sensitive to UV, visible orIR radiation.

Also shown in FIG. 1 is a camera window 145 used to package and protectthe image sensor 122. The image sensor 122 may be in a package (notshown). This package may include the window 145 through which lightpasses to reach the detector array.

The sensor 122 also may be located at the focal plane of the secondmirror 118. The sensor 122 may also be located at the conjugate imageplane to the slit 112 established by the optics (e.g., the first relaymirror 116, the grating 120, and the second relay mirror 118).Accordingly, the distance from the second mirror 118 to the sensor 122may be between about 30 mm to 60 mm in some embodiments. The sensor 122may be located elsewhere as well. Although not shown in FIG. 1, thesensor 122 may be mounted to the output block 140 in some embodiments.As discussed above, the sensor 122 may be located at an image planeconjugate to the slit 112.

In certain preferred embodiments, the sensor 122 is in communicationwith the imaging processing computer 146 such that intensity spectrumsmay be recorded. This image processing computer 146 may be incommunication with other devices including but not limited to displaydevices, storage media, or other computing or processing apparatus.

The spectrometer components, e.g., the main body 110, possible theimaging optics 144, sensor 122, and/or image processing computer 146,can be included in or mounted on a housing (not shown). The spectrometerhousing may include sockets, threaded holes, bolts, brackets, clamps,and/or other fastening arrangements for mounting, for example, in acompartment or bay or other appropriate location. The housing mayfurther include a connection including power and electrical signalswhich can be coupled to the sensor 122, scanning actuators, computer orprocessor 146, cooler, etc., if present.

This housing may protect the spectrometer components. The spectrometer100 may be included in satellites, airplanes/helicopters, unmannedaerial vehicles or on other platforms as well, such as in boats, ships,trucks, cars, balloons, rockets, or other vehicles. The spectrometers100 may be located elsewhere such as in stations (e.g., weather orresearch stations, buoys, etc.) in the field, in laboratories, inmanufacturing plants, and in medical facilities. The location and use ofthese spectrometers is not limited.

As described above, the spectrometer 100 may comprise an imagingspectrometer that produces wavelength spectrums for different spatiallocations in a two-dimensional field. The two-dimensional image fieldcan be imaged by the imaging optics 144 onto the slit 122. The slit 144can selectively pass one swath across the two-dimensional image at atime. Spectrums for a plurality of sites along the swath can be producedas a result of the wavelength dispersion of the grating 120. Thesespectral distributions are mapped onto the sensor 122 by the secondmirror 118. One spectral distribution may, for example, be mapped acrossa row of photodetectors in the detector array 122. Multiple rows ofphotodetectors in the sensor 122 may record the spectra for locationsacross the swath in this example. The two-dimensional image field isshifted with respect to the slit 112 to produce the next swath and thespectra of each portion of the new swath is measured in a like manner.Measurements for multiple swaths can be obtained and assembled toproduce spectra for a two-dimensional array of locations. Shifting ofthe spectrometer 100 which may be mounted on a movable platform such asa satellite, airplane, or UAV may permit multiple swaths to be obtained.In other embodiments, the object may be moved with respect to thespectrometer 100 in other ways. In certain embodiments, for example,movable optics coupled to a processor controlled actuator may beincluded to sweep through multiple swaths.

In addition to being configured to provide imaging, the opticalcomponents 112, 116, 120, 118 may yield well-corrected imaging. Asdescribed above, the first and second mirrors 116, 118 and the grating120 may have substantially the same center of curvature as discussedmore fully below. Additionally, the radius of curvature of the first andsecond mirrors 116, 118 may be substantially the same and may be aboutone-half the radius of curvature of the grating 120. In certainembodiments, however, the first and second mirrors 116, 118 and thegrating 120 may be nearly concentric. The centers may be slightlyoffset. Such a design provides for improved imaging.

FIG. 5 schematically illustrates such an embodiment where opticalcomponents 500 comprising concentric surfaces are used to produce awell-corrected image of an object with substantially reduced opticalaberrations. The optical path through the configuration shown in FIG. 5begins at the object plane 570 from which light propagates to a firstcurved reflector 550. The first curved reflector 550 reflects the lightto the curved reflection grating 540 where the light is diffracted andredirected to the second curved reflector 560. The second curvedreflector 560 reflects the light to the image plane 580.

The reflection grating 540 has a spherical, convex surface defined by asphere 520 with radius R1. Similarly, both of the curved reflectors 550and 560 are spherical concave mirrors whose surfaces are defined by asecond sphere 530 with radius R2. Thus, reflectors 550 and 560 havepositive power and cause the light passing through the system 500 andincident thereon to converge. Conversely, the reflection grating 540 hasnegative power and causes the light incident thereon to diverge, asshown. The distance from the object plane 570 to the first curvedreflector 550 and from the second curved reflector 560 to the imageplane 580 can be approximately equal to the focal length of the curvedreflectors 550 and 560. In certain preferred embodiments the radius R2of the second sphere 530 is approximately twice the radius R1 of thefirst circle 520. Additionally, the sphere 520 and 530 are substantiallyconcentric about a shared center 510.

The configuration illustrated in FIG. 5 has many advantages from theperspective of an optical imaging system. For example, the illustratedconfiguration of optical components 500 is characterized by unitymagnification which may help to eliminate distortion of the object imageformed at the image plane 580. Furthermore, the exact curvatures of thereflection grating 540, the first curved reflector 550, and the secondcurved reflector 560 can be chosen to display excellent spatial imagingcharacteristics over the entire image plane 580 by correcting the imagefor astigmatism and field curvature. Such correction can be accomplishedby choosing the curvatures such that the reflection grating 540 and thecurved reflectors 550 and 560 each substantially compensate for opticalaberrations introduced by one another. Field curvature can thereby bedecreased. Chromatic aberration is also reduced as reflecting opticalelements are employed. Finally, the configuration of optical elements500 is relatively simple to manufacture due to the spherical surfaces ofthe optical components.

The spectrometer 100 in FIG. 1 may be configured according to the designin FIG. 5. In particular, the first and second mirrors 116, 118 and thegrating 120 may be substantially concentric. The radii of curvature ofthe first and second mirrors 116, 118 may be substantially the same.Additionally, the radius of curvature of the grating 120 may be aboutone-half the radii of curvature of the first and second mirrors 116,118. Substantial aberration reduction may thereby be provided.

FIGS. 1 and 5 illustrate one possible configuration of optical elements.Many other configurations are possible. Additionally, the design maydeviate from perfectly concentric. For example, the center points ofspheres 520 and 530 can be offset by as much as 3 mm. This offset maycorrespond, for example, to as much as about 15% of the radius ofcurvature, R1. In certain embodiments, the offset is in a directionparallel to the z-axis shown in FIG. 5. Similarly, the length of theradius R2 of sphere 530 can deviate from twice the length of the radiusR1 of sphere 520 by as much as 10% of R2. Such offsets may improveperformance in certain embodiments. The center of curvatures of thefirst and second mirrors 116, 118 can also be offset by as much as 5 mm.Similarly, the radii of curvatures of the first and second mirrors 116,118 can be different by as much as 10%. Values outside these ranges arealso possible.

Spectrometer designs other than those specifically recited herein arealso possible. For example, additional optical and mechanical componentscan be incorporated within the instrument 100. In certain embodiments itis also possible to exclude or substitute one or more of the componentsillustrated in FIG. 1. Similarly, different arrangements andconfigurations may be used. Different shapes, sizes, and materials maybe employed.

For example, an embodiment of the invention may incorporate a singlemirror rather than the two mirrors 116, 118 illustrated in FIG. 1. Insome embodiments, the mirrors may be excluded. Conversely, additionalmirrors may be added. The mirror or mirrors 116, 118 as well as thegrating 120 may be shaped differently. The mirrors 116, 118 may beconvex or planar. Aspheric, cylindrical, or other shapes are alsopossible. Similarly the grating 120 may be concave or planar in otherembodiments. The grating 120 may be aspheric or cylindrical. Asdescribed above, the aperture 112 may comprise a slit, may be a circularor point aperture, or have other shapes. In some embodiments, theaperture 112 may be excluded.

Also as discussed above, the aperture 112 need not be formed on the turnblock 124. In some embodiments, for example, the aperture 112 may beformed on the plano-convex block 132. The grating 120 may also be formedon the plano-convex block 132 in certain embodiments; however, formingthe grating on a concave surface offers manufacturing advantages. Theaperture 112, mirrors 116, 118, and the grating 120 need not be formeddirectly on the surface of the main body 110, e.g., turn block 124 andthe meniscus block 126. Separate structures may be used for the aperture112, one or more of the mirrors 116, 118, and the grating 120 in otherembodiments and these components may be spaced apart from the main body110. Index matching may be provided in some embodiments. In someembodiments, the first and second curved mirrors 116, 118 can be spacedapart from (but on a first side of) the meniscus block 126 and thegrating 120 can be spaced apart from (but on a second side of) themeniscus block. Still other configurations are possible.

The main body 110, for instance, can be configured differently. Forexample, the main body 110 may be shaped differently and may comprisedifferent components. For example, any one of the turn block 124,plano-convex block 132, meniscus block 126, and output block 140 can beexcluded, split into more than one portion, or shaped differently. Forexample, the meniscus block 126 can be replaced with a block havingplanar surfaces rather than curved surfaces 128, 130. Similarly, theplano-convex block 132 can have a planar surface instead of a curvedsurface 134. Alternatively, the shape of the curved surfaces can bechanged, for example, from concave to convex or convex to concave or mayhave other shapes as well. Aspheric, cylindrical, or other shapes may beused in some embodiments. Additionally, curvature may be added in somecase. For example, the flat surface 136 on the plano-convex block 132may be non-flat. Similarly, the surfaces (e.g., front surface, surface138, and reflective surface 114) on the turn block 124, the secondsurface 136 on the plane-convex block 132, as well as the surfaces 142,143 on the output block may be other than flat. These surfaces may becurved to include optical power in some embodiments or may be matchedwith complementary surfaces.

Additionally, the main body 110 may comprise more or fewer portions. Forexample, the meniscus-block 126 can be split up into more than one part.Similarly, any of the plano-convex block 132, the turn block 124, andthe output block 140 can be split into two or more sections. Thesesections may be bonded together and index matched in certainembodiments. One or more of the portions 126, 132, 124, 140 of the mainbody 110 described with reference to FIG. 1 can also be excluded. Theoutput block 140 may be excluded and light may propagate through air tothe sensor 122. Alternatively, the plano-convex block 132 may be shapeddifferently to provide additional material through which the lightpropagates to the sensor 122. The turn block 124 may be excluded (orarranged so as not to provide a turn.) In some embodiments, themonolithic structure 110 may comprise simply a single portion such asthe meniscus block 126 or a differently shaped portion that replaces themeniscus block. A grating and one or more mirrors or a slit may beintegrated together with this portion. The sensor 122 may also beincluded.

While glass and, in particular, fused silica have been disclosed as asuitable substrate material, this does not preclude the use of othermaterials. Other materials substantially optically transmissive to thewavelength for which the spectrometer is to operate may be used. Othermaterials may be chosen based on their mechanical and opticalproperties, such as rigidity, coefficient of thermal expansion,transparency in the selected band of wavelengths, and index ofrefraction.

Additionally, in certain embodiments, different portions of the mainbody 110 may comprise different materials. For example, the meniscusblock 126 may comprise a first material and the plano-convex block 132may comprise a second different material. These materials may be closelyindex matched in certain embodiments and may have coefficients ofthermal expansion that yield reduced movement of the optical componentswith temperature variation. The concentric design and other designs mayprovide compensation for thermal stresses and thermal expansion in someembodiments.

Different techniques may be used to connect the different portions ofthe main body 110. As described above, an adhesive or cement may be usedto bond the different blocks 124, 132, 124, 140 together. In certainpreferred embodiments, these adhesives or cements are substantiallyoptically transmissive to the wavelength of operation and may be have anindex similar to that of the blocks to provide index matching. In someembodiments, the blocks contact one another directly and are held inplace using methods other than an adhesive. These blocks 124, 132, 124,140 may be mounted on a structure that holds the blocks in place. Indexmatching fluid or material may be disposed between the blocks 124, 132,124, 140. In other embodiments the blocks 124, 132, 124, 140 may beotherwise fused together.

Some benefits and advantages of the embodiments of the present inventiondescribed above include rigidness and compact size. As discussed,certain preferred embodiments of the invention have a substantiallymonolithic design where the optical components of the spectrometer 100can be disposed on the surface of or embedded in an opticallytransmissive material, for example fused silica. Because fused silica isa rigid, durable material, the spectrometer 100 exhibits good durabilityand rigidity. This characteristic presents a significant advantage interms of alignment of the optical components comprising the instrument.In some embodiments, for example, the optical components of thespectrometer 100 can be fabricated on a surface of one or more blocks offused silica substrate. The blocks can then be carefully aligned and maybe bonded in place. Once bonded together, the instrument acts as asingle body of material and can therefore be substantially resistant tothe effects of vibration and mechanical shock which might otherwisedisrupt the precise optical alignment of the spectrometer instrument100. Fused silica also has a low thermal expansion coefficient, makingfor a temperature stable design.

In contrast, other spectrometers may instead employ mechanical alignmentmechanisms in the optical mounts for each component, which can increasesthe size, weight, and cost of the instrument. Furthermore, thenon-monolithic design of traditional spectrometers may be prone tomisalignment during normal use and its accompanying vibrations andmechanical shocks.

Embodiments of the present invention also advantageously can be designedto have a relatively compact size without necessarily sacrificingimportant optical performance. Though the size of traditionalspectrometers can be decreased by incorporating smaller opticalcomponents, decreasing the usable aperture of the instrument canadversely affect its light-gathering power which may have a detrimentalaffect on the speed of the instrument and the resolution of the imageproduced. As has been previously discussed, embodiments of the inventionalleviate the problem associated with reducing the aperture size of theoptical components in the spectrometers by incorporating a substantiallyoptically transmissive material such as fused silica in a majority ofthe optical path inside the instrument. A spectrometer laid out in fusedsilica increases the acceptance angle of light entering the instrument.The optical throughput of a spectrometer in fused silica (or otheroptically transmissive material having an index of refraction greaterthan that of air or vacuum) is higher than the optical throughput of alike-sized spectrometer where the optical path comprises air or avacuum. Use of a higher refractive index medium allows for the design ofa smaller instrument without sacrificing optical throughput.

Accordingly, since the F-number of light increases as it enters theinstrument 100, the instrument can be designed to have a relatively higheffective F-number with smaller diameter optical components while stillaccepting an amount of light comparable to a larger instrument with alower effective F-number. Thus, utilizing an optically transmissivematerial (e.g., glass) that has a high index of refraction relative tothe environment surrounding the spectrometer 100 enables a smallinstrument with optical speed comparable to that of a larger instrumentto be realizable. Small size can be an important advantage for imagingspectrometers. Since the instrument 100 has a relatively high opticalspeed for its size, exposure times of the detector array can also beshort, which can in turn allow for higher resolution scanning since thetime between scans can be shorter.

In one embodiment of the invention substantially incorporating fusedsilica throughout the optical path of the light within the spectrometer,a compact imaging spectrometer 100 is obtained that is approximately 50mm long, 40 mm wide, and 22 mm high. In some embodiments of theinvention, materials with indexes of refraction greater than that offused silica may be used to further increase light throughput andachieve even smaller designs.

This strikingly small spectrometer instrument 100 can be used in a widerange of applications. As discussed above, the spectrometer instrument100 may be used for military, research, manufacturing, medical, andother applications. The spectrometers 100 may be located in stations(e.g., weather or research stations, buoys, etc.) in the field, inlaboratories, in manufacturing plants, in medical facilities, etc. Thespectrometer 100 may be included in satellites, airplanes andhelicopters, unmanned aerial vehicles or on other platforms as well,such as in boats, ships, trucks, cars, balloons, rockets, or othervehicles. The location and use of these spectrometers is not limited.

The spectrometer 100 may be used in the Visible-Near IR band (400-1000nm), the Short Wave Infrared (SWIR) band (900-2500 nm), the MidwaveInfrared (MWIR) band (3-5 microns), and the Longwave Infrared (LWIR)band (8-12 microns). Furthermore, spectrometers designed for use inother spectral bands are possible as well. For example, embodiments ofthe invention could be designed for use in the UV band.

Various embodiments of the invention have been described above. Althoughthis invention has been described with reference to these specificembodiments, the descriptions are intended to be illustrative of theinvention and are not intended to be limiting. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined in theappended claims.

1. A spectrometer comprising: a body comprising a mass of substantially optically transmissive material; a first reflector; a curved reflective grating, said first reflector and said curved reflective grating defining a first optical path therebetween, said first reflector and said reflective grating disposed with respect to said body such that said first optical path substantially comprises said substantially transmissive material; and a detector defining a second optical path extending from said curved reflective grating to said detector, said second optical path substantially comprising said substantially transmissive material.
 2. The spectrometer of claim 1, further comprising a second reflector disposed in said second optical path from said curved grating to said detector such that light diffracted from said curved reflective grating is reflected from said second reflector to said detector.
 3. The spectrometer of claim 2, wherein said first and second reflectors have a concave curvature defined by substantially equal radii of curvature and substantially concentric centers of curvature.
 4. The spectrometer of claim 3, wherein said reflective grating has a convex curvature defined by a radius of curvature that is about one-half that of said radii of curvature for said first and second reflectors and has a center of curvature that is substantially concentric with said first and second reflectors.
 5. The spectrometer of claim 1, further comprising a slit, light passing through said slit propagating to said first reflector and along said first optical path to said curved grating.
 6. The spectrometer of claim 1, wherein said substantially transmissive material comprises fused silica.
 7. The spectrometer of claim 1, wherein said first reflector is disposed on a surface of said body.
 8. The spectrometer of claim 1, wherein said reflective grating is disposed on a surface of said body.
 9. The spectrometer of claim 1, wherein said reflective grating has a convex curvature facing said first and second optical paths.
 10. The spectrometer of claim 1, wherein said first optical path has an optical path length between about 1 centimeters and 5 centimeters.
 11. The spectrometer of claim 1, wherein said body has a width between about 5 centimeters and 10 centimeters.
 12. The spectrometer of claim 1, wherein said detector comprises a two-dimensional CMOS detector array, a two-dimensional CCD detector array, a two-dimensional HgCdTe detector array, or a two-dimensional InSb detector array.
 13. A spectrometer comprising: a body comprising a mass of substantially optically transmissive material; a first curved reflector; and a reflective grating, said first curved reflector and said reflective grating defining a first optical path therebetween, said first curved reflector and said reflective grating disposed with respect to said body such that said first optical path substantially comprises said substantially transmissive material, wherein said reflective grating is configured to reflect broadband light having a bandwidth of at least about 400 nanometers to a detector via a second optical path comprising said substantially transmissive material.
 14. The spectrometer of claim 13, further comprising said detector.
 15. The spectrometer of claim 13, further comprising a second reflector disposed in said second optical path from said reflective grating to said detector such that light diffracted from said reflective grating is reflected from said second reflector to said detector.
 16. The spectrometer of claim 15, wherein said first and second reflectors are concave and have a curvature defined by a first sphere, said reflective grating is convex and has a curvature defined by a second sphere, and said first and second spheres are substantially concentric and the radius of said first circle is approximately twice that of said second circle.
 17. The spectrometer of claim 13, further comprising a slit, light passing through said slit propagating to said first curved reflector and along said first optical path to said grating.
 18. The spectrometer of claim 13, wherein said first curved reflector is disposed on a surface of said body.
 19. The spectrometer of claim 13, wherein said reflective grating is disposed on a surface of said body.
 20. The spectrometer of claim 13, further comprising a second curved reflector, said first and second curved reflectors defined by substantially same centers and radii of curvature, said reflective grating having a curvature defined by a center of curvature substantially concentric with said centers of curvature of said first and second curved reflectors and having a radius of curvature about one-half the radii of curvature of said first and second curved reflectors.
 21. A spectrometer comprising: a first body portion comprising substantially optically transmissive material; first and second reflective regions disposed on a first side of said first body portion; and a reflective grating disposed on a second side of said first body portion; wherein said first and second reflective portions and said reflective grating are arranged with respect to each other such that broadband light at least about 300 nanometers in bandwidth propagating through said substantially optically transmissive material incident on said first reflective region is reflected to said grating, diffracted from said grating through said substantially optically transmissive material to said second reflective portion, and reflected from said second reflective portion through said optically transmissive material.
 22. The spectrometer of claim 21, further comprising a slit, light passing through said slit propagating to said first reflective region.
 23. A spectrometer comprising: a medium comprising substantially optically transmissive material; a slit; and a reflective grating, said slit and said reflective grating defining a first optical path therebetween, said slit and said reflective grating disposed with respect to each other and said medium such that said first optical path substantially comprises said substantially transmissive material.
 24. The spectrometer of claim 23, further comprising a reflector disposed in said first optical path from said slit to said reflective grating such that light propagating though said slit is reflected from said reflector to said reflective grating.
 25. The spectrometer of 23, further comprising a detector in a second optical path extending from said reflective grating to said detector, said second optical path substantially comprising said substantially transmissive material.
 26. The spectrometer of claim 25, further comprising a reflector disposed in said second optical path from said reflective grating to said detector such that light diffracted from said grating is reflected from said grating to said detector.
 27. A spectrometer configured to be mounted in an unmanned airborne vehicle, the spectrometer comprising: a body comprising a mass of substantially optically transmissive material; a first reflector; a reflective grating, said first reflector and said reflective grating defining a first optical path therebetween, said first reflector and said reflective grating disposed with respect to said body such that said first optical path substantially comprises said substantially transmissive material, said reflective grating being configured to reflect light to a detector via a second optical path comprising said substantially transmissive material; and a housing in which said body, said first reflector, and said reflective grating are positioned, wherein said housing is no greater than 1000 cubic centimeters, and said housing is configured to be mounted in an unmanned airborne vehicle receiving area.
 28. The spectrometer of claim 27, wherein said first reflector is curved.
 29. The spectrometer of claim 27, further comprising said detector.
 30. The spectrometer of claim 27, further comprising a second reflector disposed in said second optical path from said reflective grating to said detector such that light diffracted from said reflective grating is reflected from said second reflector to said detector.
 31. The spectrometer of claim 30, wherein said second reflector has a substantially same curvature as said first reflector defined by a radius and center of curvature.
 32. The spectrometer of claim 31, wherein said grating is curved and has a center of curvature substantially concentric with said center of curvature of said first and second reflections and a radius of curvature that is about one half the radius of curvature of said first and second reflectors.
 33. The spectrometer of claim 27, wherein said spectrometer weighs no more than 2 pounds.
 34. The spectrometer of claim 27, wherein said spectrometer is configured to operate with light having a wavelength within the range of between about 400-1000 nanometers.
 35. The spectrometer of claim 27, wherein said spectrometer has a minimum spectral resolution of less than about 5 nanometers.
 36. The spectrometer of claim 27, further comprising a slit, light passing through said slit propagating to said first reflector and along said first optical path to said reflective grating.
 37. The spectrometer of claim 27, wherein said first reflector is disposed on a surface of said body.
 38. The spectrometer of claim 27, wherein said reflective grating is disposed on a surface of said body. 